Which of these numbers is prime? ${39,\ 45,\ 63,\ 79,\ 90}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 39 are 1, 3, 13, and 39. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 79 are 1 and 79. The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. Thus, 79 is a prime number.